High molecular weight polyolefins possess physical properties such as high abrasion resistance, high impact strength and low coefficient of friction. Therefore, high molecular weight polyolefins in various forms such as fibre, sheet, bio-material, wire and cable are in high demand. Polyolefins with molecular weight higher than one million g/mol, usually referred to as Ultra-high Molecular Weight Polyolefins are gaining popularity for versatile applications in various areas ranging from bio-medicals to ballistic materials. Ultra High Molecular Weight Polyethylene wherein the polymer chains are least entangled (maximum disentanglement of chains) exhibits porous morphology, high crystallinity and interesting solid state applications below melt temperature, which makes it suitable for biomedical applications such as polymeric supports in the three-dimensional regeneration and substitution of tissues by artificial prosthesis, besides applications in defence, requiring high impact strength.
Polyolefins are known to be manufactured using transition metal-catalysed polymerization technology. Typically, the catalysts used in manufacturing polyolefins are usually multi-site heterogeneous Ziegler-Natta catalysts. With the advancement in polymerization technology, single-site catalysts or group (IV) metallocene catalysts are increasingly being used for manufacturing polyolefins. Single-site catalysts and metallocene catalysts are continuously being designed to suit vital manufacturing process requirements. The improvement in the catalyst is carried out to achieve zero or near zero fouling of the equipment used and maintain catalyst activity. Development of such a catalyst with composite capability to impart various desired properties to the final product requires a careful selection of catalyst complex. Further, the selection of an appropriate ligand to be complexed with the metal component of the catalyst is carried out in such a manner that it imparts intended properties to the final product.
For instance, ethylene polymerization reaction catalysed by homogeneous single-site catalyst involves electron exchange between a ligand and a metal. It has been reported in literature that the ligand structure plays a role in determining the activity as well as the stereo-specificity of catalysts. By nature, ligands are electronically flexible and thus fulfil the requirement of imparting high activity to a resulting catalyst complex besides tailoring the same for achieving desired molecular weight, porosity, morphology, bulk density and high crystallinity of the resultant polymer.
Continuous research has resulted in the discovery of a number of highly active catalysts for the polymerization of ethylene, which include phenoxy-imine ligand early transition metal complexes (FI catalysts), pyrrolide-imine ligand group IV transition metal complexes (PI catalysts), indolide-imine ligand Ti complexes (II catalysts), phenoxy-imine ligand group (IV) transition metal complexes (IF catalysts), phenoxy-ether ligand Ti complexes (FE catalysts), imine-pyridine ligand late transition metal complexes (IP catalysts), and tris(pyrazolyl)borate ligand Ta complexes (PB catalysts). Some of the prior arts are discussed herein below.
CN101280031 discloses a process for preparing catalyst system comprising a 5,5-isopropylidene-bis (3-tert-butyl-hydroxybenzaldehyde)imine ligand complexed with transition metal. The catalyst system is prepared by reacting 5,5-isopropylidene-bis(3-tert-butyl-hydroxybenzaldehyde) with mono-amine to obtain a ligand which is then complexed with a transition metal compound.
CN101089006 discloses nickel based salicylaldehyde bridged binuclear carbodiimide type compound. The nickel based catalyst disclosed in CN101089006 when used for polymerization reaction result in ethylene oligomers and polymer with molecular weight in the range limited up to 1 lakh only.
An article titled “Arene-bridged salicylaldimine-based binuclear neutral Nickel(II) complexes: Synthesis and ethylene polymerization activities” published in Organometallics 2007, 26, 617-625 discloses nickel based catalyst having [O,N]-type ligand.
Another article titled “Ethylene and propylene polymerization behaviour of a series of bis(phenoxy-imine)titanium complexes” authored by Rieko Furuyama et al, published in Journal of Molecular Catalysis A: Chemical 200 (2003) 31-42, discloses titanium based catalysts for polymerization of ethylene and propylene. The ligand obtained by reacting 3-t-butyl salicyaldehyde/3,5-di-t-butyl salicyaldehyde and mono-amine is complexed with titanium halide to prepare the catalyst.
Still another article titled “Transition Metal Complexes of Tetradentate and Bidentate Schiff bases as Catalysts for Ethylene Polymerization: Effect of Transition Metal and Cocatalyst.” authored by Marzena Bialek, et al, published in Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 47, 565-575 (2009) discloses catalyst based on transition metals such as vanadium. titanium and zirconium. The ligand complex of the catalyst is prepared by reacting ortho-phenylene diamine and salicyaldehyde which is then complexed with transition metal to obtain the catalyst.
WO2013118140 disclose chemically immobilized heterogeneous polymerization catalyst having a salicyaldehyde imine ligand complexed with transition metal compound which is supported by a functionalized inorganic support.
The article titled “A Novel Catalyst for Olefin Polymerization” by Prof. Dr. Ibrahim M. Al Najjar, published in The Saudi International Petrochemical Technologies Conference, 2011, discloses a transition metal based catalyst which employs para-phenylene diamine for the preparation of the ligand complex.
FI catalysts are also disclosed in an article titled FI Catalysts for Olefin Oligomerization and Polymerization Principles and Practice authored by Terunori Fujita
Some of the prior art catalysts are less efficient for polymerizing olefins to achieve polymers having molecular weight in the range of 1 million g/mole. Further, the rapid reaction kinetics profile exhibited by the catalysts known in the prior art under ambient temperature and high pressure, in conjunction with the living nature of some of the catalysts has posed a challenge in regulating the molecular weight of the polymer. The problem of regulating molecular weight is further aggravated in obtaining polymers having lower molecular weight in the range of 1 million g/mol to 6 million g/mol due to uncontrollable reaction kinetics. It generally increases reaction temperature much above 65° C. which if not pacified effectively leads to formation of polymer lumps in the reaction mass. This finally ends up with fouling of the polymerization unit. Thus, eventually use of catalysts known in the prior art lead to increase in the operational cost for polymerization process due to need of frequent maintenance.
Therefore, there exists a need to develop a catalyst for polymerization reactions with improved reaction kinetic control, least operational cost and molecular weight regulation properties.